Commercial spunbonded products made from polyethylene plexifilamentary film-fibril strands have typically been produced by flash-spinning from trichlorofluoromethane; however, trichlorofluoromethane is an atmospheric ozone depletion chemical, and therefore, alternatives have been under investigation. U.S. Pat. No. 5,032,326 to Shin discloses one alternative spin fluid, namely, methylene chloride and a co-spin agent halocarbon having a boiling point between -50.degree. C. and 0.degree. C. As pointed out in Kato et al. U.S. Pat. No. 5,286,422, the Shin methylene chloride-based process is not entirely satisfactory, and the '422 patent discloses an alternative, specifically, a spin fluid of bromochloromethane or 1,2-dichloroethylene and a co-spin agent of, e.g., carbon dioxide, dodecafluoropentane, etc.
Published Japanese Application J05263310-A (published Oct. 12, 1993) discloses that three-dimensional fiber favorable for manufacturing flash-spun non-woven sheet may be made from polymer dissolved in mixtures of spin agents where the major component of the spin agent mixture is selected from the group consisting of methylene chloride, dichloroethylene, and bromochloromethane, and the minor component of the spin agent mixture is selected from the group consisting of dodecafluoropentane, decafluoropentane, and tetradecafluorohexane. However, it is known, for example, that methylene chloride is an animal carcinogen and dichloroethylene is somewhat flammable.
U.S. Pat. No. 5,023,025 to Shin discloses a process for flash-spinning plexifilamentary film-fibril strands of fiber-forming polyolefin from a group of halocarbon liquids that present a greatly reduced ozone depletion hazard. The patent discloses 1,1-dichloro-2,2,2-trifluoroethane (HCFC-123) as a preferred halocarbon (halogenated hydrocarbon). HCFC-123 is a very good spin agent for polypropylene but not for polyethylene, and in the latter case a very high spinning pressure would be required. As such, for use with polyethylene, a co-spin agent has to be employed that is capable of dissolving polyethylene at relatively low pressures (i.e., a strong solvent). The '025 patent also discloses dichlorodifluoroethane (HCFC-132b and its isomers) and dichlorofluoroethane (HCFC-141b and its isomers), all of which have significant disadvantages. For example, HCFC-132b is a good spin agent, but toxic. HCFC-141b is also a good spin agent, but somewhat flammable, and moreover exhibits a relatively high ozone depletion potential. However, regardless of any of their apparent advantages, the aforementioned spin agents all exhibit some amount of ozone depletion potential.
Flashspun products have typically been made from polyethylene. However, it is known that both polypropylene and polymethylpentene have higher melting points than does polyethylene and as such provide a flashspun product usable at higher temperatures when compared to product made from polyethylene. Moreover, certain solvents may dissolve polypropylene or polymethylpentene, but not polyethylene, therefore motivation exists to find solvents that are particularly suited to polypropylene and polymethylpentene and yet satisfy the need for non-flammability and zero or extremely low ozone depletion potential.